Currency verification heads are commonly used in daily life in applications such as vending machines, bank note counting, and counterfeit detection. Typically, banknotes use magnetic ink and other magnetic security features that produce a very weak magnetic signal. Therefore, the sensor used in a currency verification head needs to have high sensitivity, low noise, and high signal to noise ratio.
FIG. 1 shows existing currency verification head technology, which detects the perpendicular component of the magnetic field, produced by the magnetic ink on the paper currency. Typically, the material used is indium-antimonide (InSb). This type of sensor has the following disadvantages:
(1) Low sensitivity and low signal-to-noise ratio;
(2) InSb Hall Effect sensors are strongly dependent on temperature, thereby requiring complex circuitry to compensate;
(3) The magnetic biasing device must be selected to produce sufficient magnetic field in order to generate a signal with magnitude that is larger than the noise inherent in the sensor, in order to overcome the signal-to-noise limitations of the Hall effect sensor. Thus, when using Hall Effect sensors magnetic currency verification head tend to be large;
(4) A magnetic field produced by a monolithic magnetic biasing device with one pole on a flat front surface has an in-plane component that makes detection of the field produced by the paper currency difficult.
Therefore, a need exists to develop a currency verification head to accurately measure this field produced by the paper currency.